Power Supply Industry NewsKnowledge is Power ™

I have a problem with my smps. It creates audio sound when I power it up, especially at minimum load. As I increase the load then sound disappears and in some designs it is still there. I don't know what to do. It is a bottleneck for me. Anyone who knows what to do about it. I'm operating it at 50KHz which is well above audio frequency.

Hi, I have noticed that most people avoid going to article link so I have pasted here the article done so far. One point that I made worked with eliminating noise is to connect output and input ground with a coupling capacitor and other that also worked to some level was connecting a bypass capacitor in parallel to current sense resistor. This eliminated noise from my flyback design. However for my another multiple output fullbridge power supply only coupling capacitor between input and output ground was effective but did not removed noise to reasonable level. I wonder that in one SMPS that came to my observation for repairing in flyback topology had input live DC bus connected with output ground via coupling capacitor but that make sense I think so.

Acoustic noise can be a troublesome thing for a novice designer. However in order to remove it from a prototype, better understanding and sound knowledge of magnetic components is required. While troubleshooting such cases the reasons behind acoustic noise generation that I found in flyback and forward type topologies are as below.

At light loads the current sensing circuitry measures the low current demand and lowers the duty ratio. In real scenarios this duty ratio is so small that almost diminishing. Thus in next cycle a high primary current flows through mosfet and current sensing circuitry. This produces more than necessary power at output and thus in next cycles the cycle is skipped or duty ratio is lowered too much. One thing to be noted that this primary current when flowing at low output power is still at peak primary current level determined and limited by current sensing circuitry. Also this current is at random cycles. This high level of random current produces high peak flux in transformer core which causes audible noise due to magnetostriction. The solution is that we can remove acoustic noise almost if we can keep primary current level to a lower level instead of at peak cutoff level at low output power. Then if possible we c

Author

Azeem

Date

3/12/2014 8:19:09 AM

Response topic

Acoustic noise

Message

Hi,Further details are in my article at website

https://sites.google.com/site/electrophysics/articles/acoustic-noise

Thank you

Author

Azeem

Date

12/21/2013 1:48:04 AM

Response topic

Acoustic noise from SMPS

Message

Finally, I resolved audio noise problem generated from SMPS either operated in forward or flyback mode. To rectify the problem I redesigned a SMPS on vero board. It is operating at 50KHz but no audible noise from transformer detected at no load to full load. First of all I did as before and there was audible noise of around 1.6KHz :) .It was designed using same, an old traditional IC that I raised question in the discussion topic. I used the same type of snubber and clamp "ceramic" capacitors and it was not the culprit thoug I don't recommend them. The things that I did to rectifiy this problem were as below.1) Don't use single op-amp part as feedback with opto-LED cathod side connected to its output side (and anode connected to VCC side) to save another op-amp part. Even if used two op-amp parts so that signal level could be inverted again the acoustic noise would be still loud there and next suggestions should be followed.2) The feedback op-amp and connected opto-LED should be given power from much higher than 5V or other intelligent methods should be used so that it could occupy full range of current from this opto-LED.3) Correct the opto-transistor current according to current transfer ratio, CTR and problem resolved!!!!

Some other ICs like NCP120x series have resolved light and heavey load current adjustment issue that I mentioned earlier at current sense resistor to reduce or forbid the audible noise completely.

Author

Azeem

Date

1/29/2013 5:56:12 AM

Response topic

Audio Noise

Message

Now I'm in better position to tell the right reason. At light loads the current sensing circuitry measures the low current demand and lowers the duty cycle. In real scenarios this duty is so small that almost diminishing. Thus in next cycle a high primary current flows through mosfet and current sensing circuitry. This produces more than necessary power at output and thus in next cycles the cycle is skipped or duty cycle is lowered too much. One thing to be noted that this primary current when flowing at low output power is still at peak primary current level. Also this current is at random cycles. This high level of random current produces audible noise. The solution is that we can remove audible noise almost if we can keep primary current level to a lower level instead of at peak cutoff level at low output power. Then if possible we can also operate this SMPS in burst mode without danger of audio noise.

@SMPS Editor: I had later started a almost similar thread on this forum with some other valuable suggestions given by others but its deleted may be mistakenly. If possible make it available online.

Author

Azeem

Date

12/11/2012 1:01:41 AM

Response topic

audio noise

Message

Can you share what is the application of your SMPS and what topology you are using. Most of this cases of audible noise is more relevant to self-oscillating flyback which has a variable frequency between load conditions. I am speculating that when you are operating at no load, you may have small duty cycle which tends the loop to go into a burst mode condition that causes a frequency close or within the audio frequency.

For a quick check, I suggest you may want to add some pre-load on your output to introduce a minimum duty cycle that the loop can see in order for it not to operate at burst mode.

you may drop me an email at realreds2002@yahoo.com, if you want to discuss it more.

Author

Realreds

Date

9/25/2008 1:36:58 PM

Response topic

Audio noise

Message

Dear AzeemI think I can help you send your design plans to mo_mesbah@yahoo.comYou know, the source of noise is always inductors and transformers. the cause is the voltage not the current! at noload you may have higher voltage values! check it and send the results to the email address.good luck

Author

Mo

Date

9/16/2008 4:24:33 AM

Response topic

Try a shunt regulator.

Message

Power supply rail noise is a problem in audio circuits. One method of combating this is to use a shunt regulator. This problem usually occurs in the preamplifier stages as the preamplifier usually picks up and amplifies the power supply rails. In most cases a series pass regulator such as a 7815 / 7915 IC regulator is used, and because of the feedback loops inside these IC's, to maintain voltage regulation a line ripple and white noise is introduced to the supply line. A similar effect would apply when using a SMPS. Shunt regulators are basically current regulators where the pass element is in parallel, with the supply rails and is set to maintain a constant current. The arrangement is as follows: (POWER SUPPLY) - (SERIES RESISTOR (low ohm: 2ohm, 10watt))- (PARALLEL PASS ELEMENT) - (FILTER CAPACITORS) - (AMPLIFIERS).

Author

D.Brown

Date

9/7/2008 11:39:58 PM

Response topic

audio noise

Message

Dear Azeem,

Please check whats the minimum duty cycle your control chip can handle. If the duty cycle goes below that, you are going to see some cycle skipping. As your switching frequency is 50kHz, your duty cycle is going below 40% of the minimum duty cycle and hence results in an equivalent switching frequency below 20Khz going into audio frequency zone.

Please check the control chip minimum duty cycle.

Author

Dr. Manoj

Date

9/1/2008 12:17:53 AM

Response topic

audio noise

Message

I have noted that when load is at maximum and there is no audio noise the waveshape at mosfet drain is very clean. When load is at minimum, the waveshape at mosfet drain is distorted in a mannar that there is a time when there is no duty cycle or zero duty cycle. This phenomena i.e. when there is some dutycycle and the time when there is no dutycycle is in the audio range which creates audio noise. I think at that time it is operating in discontinuous mode. I have also tried the feedback loop type-II error amplifier in a different fashion and that has some effect on the audio noise. Also can u tell me what to do with this audio noise if I don't want to add a minimum load at output.

1. Check if your power supply is in continuous or discontinuous mode as suggested.2. Get the open loop transfer function when its in oscillation mode. 3. Using your Type-II controller, find the closed loop phase and gain margins. Its probable that you don't have sufficient phase and gain margins for low load conditions.

This should give better picture whats going on. If you face problem in getting the same, please email me on the email address given below

Dr. Manojrcss_enerzies@yahoo.com

Author

Dr. Manoj

Date

8/27/2008 11:27:29 PM

Response topic

audio noise

Message

I'm operating it in forward topology. Input is 220VAC and output around 41VDC. Output current 1.5A max only. I'm using type-II error amplifier op-amp based for loop stability. If u can help me then send my your email address and I will email u my schematic.

Author

Azeem

Date

8/27/2008 12:13:44 AM

Response topic

Audio Noise

Message

It looks like your power supply oscillate at light load. The inductor may go to discontinuous mode. Check your loop stability.

Author

David Do

Date

8/26/2008 7:10:54 AM

Response topic

audio noise

Message

Dear Azeem,

Can you detail little bit more about the power supply you are facing the problem. Mainly I would be interested to know the specification and topology you have used.

With RegardsDr. Manojwww.rcssgroup.com

Author

Dr. Manoj

Date

8/25/2008 10:53:01 AM

Response topic

@azeem

Message

hello, kindly forward your schematic & transformer design alon with waveforms to prasun@spectraind.comfurther, It may be the transformer capacitance which is making oscillations in the circuit. Also,better i could suggest if you could provide me the details.